1. Field of the Invention
The present invention relates to a microwave oscillator for oscillating a microwave and a millimeter-wave, in particular, to a microwave oscillator for allowing a phase noise of degrading a spectral purity to reduce.
2. Description of the Related Art
With reference to accompanying drawings, a conventional microwave oscillator will be described.
FIG. 1 is a circuit diagram showing the structure of a first example of a conventional microwave oscillator. The microwave oscillator shown in FIG. 1 is a series feedback oscillator of which a feedback short-circuited stub 2 is connected to the source terminal of a field effect transistor (FET) 1. A transmission line 11 and a capacitor 6 are connected in series to the gate terminal of the FET 1. A gate bias circuit 3 supplies a DC bias to the gate terminal of the FET 1 through a spurious oscillation suppression resistor 14 and a transmission line 11. A drain bias circuit 4 supplies a DC bias to the drain terminal of the FET 1 through a transmission line 11.
Each of the bias circuits 3 and 4 is composed of a 1/2 wavelength line 5, a capacitor 6 short-circuited in a high frequency band region, and a DC power supply 7. An oscillated output signal is extracted from a load resistor 10 through a transmission line 11 and an output matching circuit 8 that are connected in series to the drain terminal of the FET 1. The output matching circuit 8 is composed of a transmission line 11 and a matching open stub 12.
FIG. 2 is a circuit diagram showing the structure of a second example of a conventional microwave oscillator. The microwave oscillator shown in FIG. 2 is composed of the microwave oscillator shown in FIG. 1 and a transmission line resonating circuit 9. The transmission line resonating circuit 9 is composed of a 1/2 wavelength open stub 25. Examples of the microwave oscillator shown in FIG. 2 are disclosed in "1994 The Institute of Electronics, Information and Communication Engineers, Japan, Spring Convention C-43" and "1993 The Institute of Electronics, Information and Communication Engineers, Japan, C-44", and so forth.
FIG. 3 is a circuit diagram showing the structure of a third example of a conventional microwave oscillator. The microwave oscillator shown in FIG. 3 is composed of the microwave oscillator shown in FIG. 1 and a transmission line resonating circuit 9. In particularly, the transmission line resonating circuit 9 is composed of a capacitor 6 and a 1/4 wavelength short-circuited stub 26. The capacitor 6 is short-circuited in a high frequency region.
FIG. 4 is a circuit diagram showing the structure of a fourth example of a conventional microwave oscillator. The microwave oscillator shown in FIG. 4 has a resonating circuit 9 of which the capacitor 6 connected to the gate terminal of the FET 1 in the microwave oscillator shown in FIG. 1 is substituted with a 1/4 wavelength open stub 27. Examples of the microwave oscillator shown in FIG. 4 are disclosed in "1994 The Institute of Electronics, Information and Communication Engineers, Japan, Spring Convention C-73" and "1992 The Institute of Electronics, Information and Communication Engineers, Japan, C-60", and so forth.
FIG. 5 is a circuit diagram showing the structure of a fifth example of a conventional microwave oscillator. The microwave oscillator shown in FIG. 5 is composed of the microwave oscillator shown in FIG. 1 and a resonating circuit 9. The resonating circuit 9 is composed of a coupling transmission line 28 and a dielectric resonator 29. Examples of the microwave oscillator shown in FIG. 5 are disclosed in "IEEE Transaction on Microwave Theory and Techniques, Vol, MTT-31, p. 312" and so forth.
Although microwave oscillators having the above-described dielectric resonator 29 are disclosed in Japanese Patent Laid Open Publication Nos. Sho-61-205009, Hei-3-140003, and Hei-7-176954, these related art references do not mention the reduction of the size of the structure.
However, in the first example of the conventional microwave oscillator shown in FIG. 1, since it does not contain a resonant circuit with a sharp frequency characteristic, the quality factor Q value of the circuit is small and the phase noise thereof is large.
On the other hand, since the second to fourth examples of the conventional microwave oscillators shown in FIGS. 2 to 4 have a transmission line resonating circuit of which one of a transmission line is opened or short-circuited is disposed, the Q value of the circuit increases and thereby the phase noise reduces. These transmission line resonators can be easily structured with small planar circuits. However, since the unloaded Q value is not large, the phase noise cannot be sufficiently reduced.
On the other hand, in the fifth example of the conventional microwave oscillator shown in FIG. 5, with a resonating circuit 9 of which a dielectric resonator 29 is magnetically coupled with a coupling transmission line 28, the phase noise is reduced. Since the dielectric resonator has a very large unloaded Q value, the phase noise can be remarkably reduced. However, the size and cost of the circuit and the module become large. In addition, the fabrication cost becomes large.